1. Field of the Invention
The present invention relates to a drive control device for an electric load including a switching element serially connected to an electric load to which a power is supplied from a drive power source, and monitoring/controlling means for performing opening/closing control of the switching element, and monitoring presence/absence of an abnormal state of a power supply circuit for the electric load, and more particularly, to a drive control device for an electric load improved so as not to miss whether a switching element is correctly brought into a closed-circuit state or an open-circuit state according to a momentary circuit closing command or circuit opening command, or a change in the command from the circuit closing command to the circuit opening command or the circuit opening command to the circuit closing command thereby surely detecting presence/absence of an abnormal state.
2. Description of the Related Art
In power supply control for an electric load, an intelligent switch provided with a feature of monitoring an abnormality of a switching element, which is a power transistor, is widely used. For example, according to the “ABNORMALITY DETECTION APPARATUS FOR A POWER SUPPLY CIRCUIT” described in Japanese Patent Application Laid-open No. 2007-135294 (FIG. 1, Abstract), switching control means (corresponding to the monitoring/controlling means) is caused to generate a control output signal directed to a power transistor (corresponding to the switching element), and a state detection circuit is caused to report an open/closed-circuit state of the power transistor, as a state detection signal, to the switching control means, and the switching control means compares a result of determining whether the control output signal generated by itself is the circuit closing command or the circuit opening command and a result of determining whether the reported state detection signal is the closed-circuit state or the open-circuit state with each other, thereby determining presence/absence of an abnormality of the power supply circuit for the electric load and the switching element.
For example, when, in response to the circuit closing command, a voltage between both terminals of a power transistor is equal to or more than a predetermined value, and thus the power transistor is not in the closed-circuit state, it is determined that this state is a load short-circuit abnormality in which an electric load is short-circuited or the electric load is short-circuited by contacts of external wiring, and thus an over current is flowing, or a closed-circuit abnormality caused by an internal fusion abnormality in which the power transistor cannot close or the like. Further, when, in response to the circuit opening command, the voltage between both the terminals of the power transistor is equal to or less than a predetermined value, and thus the power transistor is not in the open-circuit state, it is determined that this state is an open circuit abnormality which is an internal short-circuit of the power transistor or both the terminals of the power transistor is short-circuited by contacts of external wiring.
However, even when, in response to the circuit closing command, the voltage between both terminals of the power transistor is equal to or less than the predetermined value, and it thus appears that the power transistor is in the normal closed-circuit state, there may be an open-circuit abnormal state caused by an internal short circuit and an external short circuit of the power transistor. Further, even when, in response to the circuit opening command, the voltage between both terminals of the power transistor is equal to or more than the predetermined value, and it thus appears that the power transistor is in the normal open-circuit state, there may be a closed-circuit abnormal state caused by an internal short circuit or an external short circuit of the electric load. Thus, only after both the normal closed-circuit state and the normal open-circuit state are detected, it is confirmed that the power supply circuit and the switching element are normal, and a state in which only the normal closed-circuit state or the normal open-circuit state is detected is a state in which a normal state is preliminary determined.